In addition to the see-and-avoid failure on the part of the colliding aircraft, seems ATC shares some blame.
1. ATC called Cessna and Metro as traffic to the Cirrus. Cirrus responded with “traffic in sight,” and ATC never followed up to verify that Cirrus had Cessna (lesser conflict) and Metro (greater conflict) in sight.
2. ATC never called Cirrus as traffic for the Metro.
3. Given the proximity of the parallel runways at KAPA, seems ATC should have some type of scheme to deconflict simultaneous visual approaches.